jsp Cannot forward after response has been committed问题,

jsp Cannot forward after response has been committed问题,
String username = request.getParameter("username");
String password = request.getParameter("password");
String confirmPassword = request.getParameter("confirmPassword");
String sql = "select username,password from users ";
QueryRunner qr = DbHelper.getQueryRunner();//工具类,数据库肯定没问题
List list = null;
User user = null;//bean类
try {
list = (List) qr.query(sql,new BeanListHandler(User.class));
} catch (Exception e) {
e.printStackTrace();
}
if (password.equals(confirmPassword)) {
for (int i = 0; i < list.size(); i++) {
user = (User) list.get(i);
if (user.getUsername().equals(username)) {
request.setAttribute("message","用户名已经被注册!");
request.getRequestDispatcher("/admin/login.jsp").forward(
request,response);
}else {
sql = "insert into users(username,password) values(?,)";
String params[] = { username,password };
qr = DbHelper.getQueryRunner();
try {
qr.update(sql,params);
} catch (SQLException e) {
e.printStackTrace();
}
request.setAttribute("message","注册成功!");
request.getRequestDispatcher("/admin/login.jsp").forward(request,
response);
}
}
}
else {
request.setAttribute("message","两次密码不一样,请重新输入");
request.getRequestDispatcher("/admin/reg.jsp").forward(request,
response);
}